CN105509383A - Refrigerant recovery in natural gas liquefaction processes - Google Patents

Refrigerant recovery in natural gas liquefaction processes Download PDF

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Publication number
CN105509383A
CN105509383A CN201510647465.0A CN201510647465A CN105509383A CN 105509383 A CN105509383 A CN 105509383A CN 201510647465 A CN201510647465 A CN 201510647465A CN 105509383 A CN105509383 A CN 105509383A
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natural gas
mix refrigerant
closed loop
refrigeration circuit
cold
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CN201510647465.0A
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CN105509383B (en
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B.K.约翰斯顿
G.克里什纳墨菲
M.J.罗伯茨
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Air Products and Chemicals Inc
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Air Products and Chemicals Inc
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/0052—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
    • F25J1/0055—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream originating from an incorporated cascade
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/008—Hydrocarbons
    • F25J1/0092—Mixtures of hydrocarbons comprising possibly also minor amounts of nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/0097—Others, e.g. F-, Cl-, HF-, HClF-, HCl-hydrocarbons etc. or mixtures thereof
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    • F25J1/0214—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle
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    • F25J1/0214—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle
    • F25J1/0215—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle with one SCR cycle
    • F25J1/0216—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle with one SCR cycle using a C3 pre-cooling cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0245—Different modes, i.e. 'runs', of operation; Process control
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/025—Details related to the refrigerant production or treatment, e.g. make-up supply from feed gas itself
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
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    • F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
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    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/62—Details of storing a fluid in a tank

Abstract

The invention relates to refrigerant recovery in natural gas liquefaction processes. Described herein is a method of removing refrigerant from a natural gas liquefaction system in which vaporized mixed refrigerant is withdrawn from the closed-loop refrigeration circuit and introduced into a distillation column so as to be separated into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components. Overhead vapor is withdrawn from the distillation column to form a methane enriched stream that is removed from the liquefaction system, and bottoms liquid is reintroduced from the distillation column into the closed-loop refrigeration circuit. Also described are methods of altering the rate of production in a natural gas liquefaction system in which refrigerant is removed as described above, and a natural gas liquefaction systems in which such methods can be carried out.

Description

Refrigerant-recovery in natural gas liquefaction process
Technical field
The present invention relates to the method removing cold-producing medium from natural gas liquefaction system, it uses the cold-producing medium of mixing to make natural gas liquefaction and/or excessively cold, and relate to the method for the productivity ratio of that change liquefaction or excessively cold natural gas, wherein cold-producing medium is closed in production or is removed from liquefaction system during idling (turndown).The invention still further relates to the natural gas liquefaction system that can perform said method.
Background technology
Many is well-known for carrying out liquefaction to natural gas with liquefaction system excessively cold alternatively in the art.Usually, in such systems, natural gas by being liquefied with the indirect heat exchange of one or more cold-producing mediums, or to be liquefied and excessively cold.In many this systems, the cold-producing medium of mixing is used as cold-producing medium or wherein a kind of cold-producing medium.Usually, the cold-producing medium of mixing circulates in closed loop refrigeration circuit, and closed loop refrigeration circuit comprises main heat exchanger, natural gas transport by this main heat exchanger, to be undertaken by the indirect heat exchange of the mix refrigerant with circulation liquefying and/or excessively cold.The example of this kind of refrigeration cycle comprises single mix refrigerant (SMR) circulation, the mix refrigerant (C3MR) of propane-precooling circulates, double-mixed refrigerant (DMR) circulates and C3MR-nitrogen mixing (such as AP-X) circulation.
During normal (stable state) operation of this system, the cold-producing medium of mixing in closed loop refrigeration circuit inner loop, and can not be removed wittingly from loop.The cold-producing medium warmed leaving the evaporation of main heat exchanger was usually compressed before returning main heat exchanger, cool, condensation at least in part; then as cold evaporation or the cold-producing medium that evaporating carry out expand (therefore closed loop refrigeration circuit also comprises one or more compressor, cooler and expansion gear usually), thus again provide cooling task for main heat exchanger.A small amount of mix refrigerant may have lost along with the time, and such as, due to the leakage that loop is medium and small, loop then may need to add a small amount of supplementary cold-producing medium, but does not usually have in the normal operation period or a small amount of cold-producing medium carries out removing or adding from loop.
But, under the condition of unfavorable (upset), such as, during the closedown or idling of liquefaction system, the cold-producing medium removing mixing from closed loop refrigeration circuit may be needed.In the down periods, under the out-of-operation condition of compressor, cooler and main heat exchanger, the temperature of the mix refrigerant of closed loop cooling circuit inside and thus pressure are by due to warming and stably raise along with the time around loop, and this then needs from loop, to remove cold-producing medium before the pressure of accumulation may cause causing the point of damage to other component any in main heat exchanger or loop.During idling, the quantity in stock adjusting mix refrigerant may be needed, correctly to mate the productivity ratio (more particularly, the cooling task amount of minimizing required in main heat exchanger) of minimizing, this needs to remove some cold-producing medium again from closed loop refrigeration circuit.
Remove cold-producing medium from closed loop refrigeration circuit to be discharged simply or burn, but cold-producing medium is of great value goods often, this makes this be undesirable.In order to avoid this problem, in this area, another selection adopted is in reservoir vessel, store the cold-producing medium removed from closed loop refrigeration circuit, makes it keep and is back in closed loop cooling circuit follow-up.But this scheme also relates to operational difficulty.The mix refrigerant removed from closed loop refrigeration circuit will still need to cool continuously usually, thus make it store with at least part of condensing state, thus avoid excessive pressure store and/or volume.The running cost that this cooling and condensation duty then may relate to great energy ezpenditure and be associated is provided.
Such as, US2012/167616A1 discloses a kind of method of the system for operating the liquefaction for gas, the closed loop refrigeration circuit that this system comprises main heat exchanger and is associated.This system also comprises cold-producing medium drum, and it is connected on main heat exchanger or forms a part for refrigerating circuit, and wherein cold-producing medium can store in the down periods of liquefaction system, thus avoids having to discharge the cold-producing medium evaporated.Storage drum is provided with heat trnasfer device (such as the heat transfer coil that auxiliary refrigerant is passed), and it is for cooling and liquefy the cold-producing medium be contained in storage drum.Main heat exchanger also can be connected on supply lines, and wherein liquid refrigerant can be directly injected in main heat exchanger, thus cooling is contained in the cold-producing medium of there.
Similarly, the document IPCOM000215855D in ip.com database discloses a kind of method prevented in down periods coil winding formula heat exchanger generation overvoltage.The mix refrigerant of evaporation is extracted out from the shell-side of coil winding formula heat exchanger, and send to the container with heat transfer coil, liquefied natural gas stream can be pumped and pass through coil pipe, or liquefied natural gas can be directly injected in coil pipe, thus the cold-producing medium of cooling and condensation mixing, then the cold-producing medium of mixing returns the shell-side of coil winding formula heat exchanger.In alternative layout, by heat transfer coil being placed on the inside of shell or being directly injected in shell by liquefied natural gas, the cooling of the mix refrigerant of evaporation and condensation can occur in the shell-side of coil winding formula heat exchanger.Liquefied natural gas stream can obtain from accumulator tank or from any point of the cold junction of liquefaction unit.
US2014/075986A1 describes and during starts utilizes a kind of main heat exchanger of liquefaction facility and closed loop refrigeration circuit to be separated the method for ethane and natural gas at liquefaction facility, but not for the production of liquefied natural gas, thus accelerating the production of ethane, ethane is about to the part being used as mix refrigerant during the subsequent normal operations of liquefaction facility.
US2011/0036121A1 describes a kind of method removing the natural gas pollutant leaked in the nitrogen cold-producing medium of circulation, nitrogen cold-producing medium in reverse Brayton cycle for liquefied natural gas.A part for nitrogen cold-producing medium is extracted out from circulation, liquefies at the cold junction of main heat exchanger, and is introduced into the top of destilling tower as backflow.The nitrogen steam of purification is extracted out, to return circulation from the top of destilling tower.The liquid comprising natural gas pollutant extracted out bottom destilling tower can be added into the liquefied natural gas stream of liquefaction system production.
US2008/0115530A1 describes a kind of method removing pollutant from the flow of refrigerant that the closed-loop refrigeration cycle of liquefied natural gas (LNG) facilities adopts.Flow of refrigerant may be the methane refrigerant or ethane refrigerant that adopt in cascade cycle, and wherein pollutant comprises the heavier cold-producing medium (such as distinguishing ethane or propane) leaked into from the cold-producing medium of the independently closed loop of cascade cycle.This system utilizes destilling tower to remove pollutant.Contaminated cold-producing medium is introduced in destilling tower in centre position.The vapor stream of the cold-producing medium cleared the pollution off is extracted out from the top of tower, and returns its closed loop refrigeration circuit.The liquid being rich in pollutant is extracted out from the bottom of tower and is dropped.
Summary of the invention
According to a first aspect of the invention, provide a kind of method removing cold-producing medium from natural gas liquefaction system, it uses mix refrigerant to liquefy to natural gas and/or excessively cold, mix refrigerant comprises the mixture of methane and one or more heavier compositions, and liquefaction system comprises closed loop refrigeration circuit, wherein mix refrigerant circulates when using liquefaction system, closed loop refrigeration circuit comprises main heat exchanger, natural gas transport passes through main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold, described method comprises:
A () extracts the mix refrigerant of evaporation out from closed loop refrigeration circuit;
B the mix refrigerant of evaporation is introduced in destilling tower by (), and provide backflow for destilling tower, thus the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane and the bottom liquid being rich in heavier composition;
C () extracts overhead vapours out from destilling tower, to form the stream being rich in methane, the stream being rich in methane removes from liquefaction system; With
D bottom liquid is introduced into closed loop refrigeration circuit from destilling tower by () again, and/or store bottom liquid, makes it follow-uply can be introduced in closed loop refrigeration circuit again.
According to a second aspect of the invention, provide a kind of method changing the productivity ratio of liquefaction in natural gas liquefaction system and excessively cold natural gas, liquefaction system uses mix refrigerant to liquefy to natural gas and/or excessively cold, liquefaction system comprises closed loop refrigeration circuit, wherein mix refrigerant circulates in closed loop, mix refrigerant comprises the mixture of methane and one or more heavier compositions, and closed loop refrigeration circuit comprises main heat exchanger, natural gas transport passes through main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold, described method comprises:
First period, pass through main heat exchanger with the first feed rate transport gas during this period, and the cold-producing medium of mixing circulates with the first cycle rate in closed loop refrigeration circuit, thus produce liquefaction or excessively cold natural gas with the first productivity ratio;
Second period, during this period, by stopping by the natural gas feeding of main heat exchanger or its feed rate being reduced to the second feed rate, stop the circulation of mix refrigerant in closed loop refrigeration circuit or its cycle rate is reduced to the second cycle rate, and from liquefaction system, remove cold-producing medium, stop the production of liquefaction or excessively cold natural gas, or the productivity ratio of liquefaction or excessively cold natural gas is reduced to the second productivity ratio, and the method wherein removing cold-producing medium from liquefaction system comprises:
A () extracts the mix refrigerant of evaporation out from closed loop refrigeration circuit;
B the mix refrigerant of evaporation is introduced in destilling tower by (), and provide backflow for destilling tower, thus the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane and the bottom liquid being rich in heavier composition;
C () extracts overhead vapours out from destilling tower, to form the stream being rich in methane, the stream being rich in methane removes from liquefaction system; With
D bottom liquid is introduced into closed loop refrigeration circuit from destilling tower by () again, and/or store bottom liquid, makes it follow-uply can be introduced in closed loop refrigeration circuit again.
According to a third aspect of the invention we, provide a kind of natural gas liquefaction system to liquefy to natural gas and/or excessively cold, natural gas liquefaction system uses the cold-producing medium of mixing, and the cold-producing medium of mixing comprises the mixture of methane and one or more heavier compositions, and described liquefaction system comprises:
Closed loop refrigeration circuit, it is for holding when using liquefaction system and making the refrigerant circulation of mixing, closed loop refrigeration circuit comprises main heat exchanger, and natural gas can be conducted through main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold;
Destilling tower, it for receiving the mix refrigerant of the evaporation from closed loop refrigeration circuit, and can operate, the mix refrigerant of evaporation be separated into the overhead vapours being rich in methane of mix refrigerant and be rich in the bottom liquid of heavier composition;
For providing the device of backflow for destilling tower;
Conduit, it is for passing to destilling tower by the mix refrigerant of evaporation from closed loop refrigeration circuit, for extracting out from destilling tower and remove the stream being rich in methane formed by overhead vapours from liquefaction system, and for bottom liquid is introduced into closed loop refrigeration circuit from destilling tower again.
Accompanying drawing explanation
Fig. 1 is schematic flow chart, which depict the natural gas liquefaction system according to an embodiment of the invention operated during the first period, wherein it operates under normal operation, and during this period, liquefaction and excessively cold natural gas are produced with the first productivity ratio or normal production rate.
Fig. 2 is schematic flow chart, which depict the natural gas liquefaction system operated during the second period now, wherein it operates under idling or closedown condition, during this period, the production of liquefaction and excessively cold natural gas is reduced or stops, and cold-producing medium removes now from natural gas liquefaction system.
Fig. 3 is schematic flow chart, which depict the natural gas liquefaction system according to another embodiment of the present invention operated during the second period equally, wherein it operates under idling or closedown condition, during this period, the production of liquefaction and excessively cold natural gas is reduced or stops, and cold-producing medium removes now from natural gas liquefaction system.
Fig. 4 is schematic flow chart, which depict the natural gas liquefaction system according to another embodiment of the present invention operated during the second period equally, wherein it operates under idling or closedown condition, during this period, the production of liquefaction and excessively cold natural gas is reduced or stops, and cold-producing medium removes now from natural gas liquefaction system.
Fig. 5 is schematic flow chart, which depict the natural gas liquefaction system according to an embodiment of the invention operated during the 3rd period, during this period, the production of liquefaction and excessively cold natural gas returns to normal operating condition, and cold-producing medium is introduced in natural gas liquefaction system again.
Fig. 6 is schematic flow chart, which depict the natural gas liquefaction system according to another embodiment of the present invention operated during the 3rd period equally, during this period, the production of liquefaction and excessively cold natural gas returns to normal operating condition, and cold-producing medium is introduced in natural gas liquefaction system again.
Detailed description of the invention
The cold-producing medium of mixing is of great value goods in natural gas liquefaction plant.Usually, they can utilize or integrated or natural gas liquids (NGL) recovery system before liquefying and carry out extracting and manufacturing from natural gas feed itself with liquefaction.But, although the composition of mix refrigerant such as methane can obtain easily by this way, but some other composition is more time-consuming and is more difficult to isolate (such as ethane/ethylene and higher hydrocarbon, it only exists a small amount of in natural gas), or may may not obtain by this way (such as HFC, it does not exist in natural gas).In fact, therefore, the heavier composition of mix refrigerant may must be input in facility with great expense.Therefore, the loss of this cold-producing medium has great economic impact.
But, comparably, for the above reasons, in adverse conditions, such as, during the closedown or idling of liquefaction system, may need to remove cold-producing medium from closed loop refrigeration circuit.The cold-producing medium removing mixing from closed loop refrigeration circuit may be discharge simply or burn, but this cold-producing medium and especially its heavier composition be have lost.Or the mix refrigerant removed can store under the state of at least part of condensation, but as mentioned above, the running cost that the cooling task for this reason needed may relate to great energy ezpenditure and be associated.
As mentioned above, according to of the present invention first, second and the method and system of the third aspect solve these problems by the mix refrigerant of the initial evaporation removed from the closed-loop refrigerant circuits of destilling tower being separated into the cut (it collects in a distillation column as overhead vapours) being rich in methane and the cut (it collects in a distillation column as bottom liquid) being rich in heavier composition, allow that the stream being rich in methane is discharged from liquefaction system, and the stream being rich in heavier composition returns closed loop refrigeration circuit and/or stores, be introduced into again in closed loop refrigeration circuit for follow-up.
In this manner, once have passed through the reason that must remove cold-producing medium, and the words of the normal operating of recovery liquefaction system, the heavier composition (such as ethane/ethylene and higher hydrocarbon) of mix refrigerant can greatly be retained, thus avoids difficulty and/or the cost that must change these compositions in mix refrigerant.Simultaneously, by removing the stream (simply by this stream of burning or by being placed on other purposes) being rich in methane formed by overhead vapours from destilling tower and liquefaction system, also avoid with until difficulty that before enabling, storing methane is associated and cost.Mention above, because methane exists as the main component of the available natural gas in scene, so the methane changed in cold-producing medium is one relatively easily and fast technique.Equally, also be present in the cold-producing medium of mixing at nitrogen, and thus also as be rich in methane stream a part and be removed when, this also relatively easily changes usually, because natural gas liquefaction system needs nitrogen for deactivation object usually, therefore scene has nitrogen generation facility often.In addition, because be present in the cold-producing medium methane of mixing, nitrogen (if present) and other light composition any will have the steam pressure higher than the heavier composition of mix refrigerant, so they need colder storage temperature (or higher pressure store) inherently, this also makes discharge but not store these compositions advantageously.
Indefinite article used herein " one " and " one " unless pointed out, otherwise mean one or more when being applied to any feature in the embodiment of the present invention described by description and claim.This restriction unless specifically stated, the use of " " and " " do not limit the connotation of single feature.The article be positioned at before odd number or plural noun or noun phrase represents the feature of special provision, and depends on its environment used and may have odd number or plural references.
Word used herein " natural gas " also comprise synthesis with the natural gas of substitute.The main component of natural gas is methane (it forms at least 85 % by mole usually, at least 90 % by mole more often, and the feed flow of about 95 % by mole of average out to).Other typical composition of natural gas comprises nitrogen, one or more other hydrocarbon and/or other composition, such as helium, hydrogen, carbon dioxide and/or other sour gas and mercury.But before experience liquefaction, the compositions such as such as moisture, sour gas, mercury and natural gas liquids (NGL) remove the level to necessity from charging, thus avoid the condensation in the heat exchanger occurred in liquefaction or other operational issue.
Unless pointed out, otherwise word used herein " mix refrigerant " refers to the synthetic comprising methane and one or more heavier compositions.It also may comprise one or more extra light compositions.Word " heavier composition " refers to have the composition than the methane more mix refrigerant of low volatility (namely more higher boiling).Word " light composition " refers to have composition that is identical with methane or more high volatile volatile (namely identical or more low boiling).Typical heavier composition comprises heavier hydrocarbon, such as but be not limited to ethane/ethylene, propane, butane and pentane.Extra or alternative heavier composition can comprise hydrocarbon (HFC).Nitrogen is also present in the cold-producing medium of mixing often, and the light composition that composition is typically extra.When it is present, nitrogen, by destilling tower and methane separation, makes from the overhead vapours of destilling tower and is also rich in nitrogen from the stream being rich in methane that liquefaction system removes.In a kind of variant, the cold-producing medium that method and system of the present invention also can be applicable to wherein mix does not comprise methane but comprises the method and system of nitrogen and one or more heavier compositions (such as N2/HFC mixture), overhead vapours from destilling tower is rich in nitrogen, and from liquefaction system, remove the stream being rich in nitrogen.But this is not preferred.
Any suitable refrigerant circulation can be adopted to carry out liquefaction and excessively cold alternatively to natural gas according to the liquefaction system in method and system of the present invention, such as but be not limited to single mix refrigerant (SMR) circulation, mix refrigerant (C3MR) circulation of propane-precooling, double-mixed refrigerant (DMR) circulation and C3MR-nitrogen mixing (such as AP-X) and circulate.Closed loop refrigeration circuit can be used for liquefying to natural gas and excessively cold, and the cold-producing medium wherein mixed circulates in closed loop refrigeration circuit, or it can only for liquefied natural gas, or carry out cold to the natural gas that the another part of the system that is liquefied liquefies.Incessantly a kind ofly hold in the system of the closed loop of mix refrigerant existing, can use in conjunction with the mix refrigerant only existed only in one of them closed loop according to the method removing cold-producing medium according to the present invention, or can use in conjunction with the mix refrigerant be present in more than or all closed loops.
Word used herein " main heat exchanger " refers to a part for closed loop refrigeration circuit, natural gas through it, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold.Main heat exchanger can be made up of one or more series connection and/or the cooling end be arranged in parallel section.Each this portion section can be made up of the separate unit with himself shell, but equal portion's section may be combined with into individual unit to share public shell.Main heat exchanger can have any suitable type, and such as but be not limited to shell and tube heat exchanger, coil winding formula heat exchanger or plate wing type, but heat exchanger is preferably coil winding formula heat exchanger.In this interchanger, each cooling end section will generally include himself tube bank (wherein interchanger has package type or coil winding type) or plate wing bundle (wherein this unit has plate wing type).Unless pointed out, otherwise " hot junction " of main heat exchanger used herein is relative word with " cold junction ", refers to that (difference) has the end of the main heat exchanger of maximum temperature and minimum temperature, and is not intended to contain any special temperature range." centre position " of phrase main heat exchanger refers to the position between hot junction and cold junction, usually between the cooling end section of two series connection.
The mix refrigerant of the evaporation of extracting out from closed-loop refrigerant circuits is preferably extracted out from cold junction and/or from the centre position of main heat exchanger.When main heat exchanger is coil winding formula heat exchanger, the mix refrigerant of evaporation is preferably extracted out from the shell-side of coil winding formula heat exchanger.
Word used herein " destilling tower " refers to tower (or one group of tower), it comprises one or more separation level be made up of device (such as filler or pallet), is separated level and adds contact and thus enhance the quality transmission between the steam and the liquid flowed downward of the rising of tower internal flow.Like this, the concentration of methane and other light composition any (such as nitrogen) is when it is present added in the steam risen, the steam risen collects in the top of tower as overhead vapours, and in the bottom liquid coming together in tower bottom, add the concentration of heavier composition." top " of destilling tower refer to be positioned at uppermost separation level or on tower section." bottom " of destilling tower refer to be positioned at bottommost separation level or under tower section.
The bottom of destilling tower is preferably introduced into from the mix refrigerant of the evaporation of closed loop refrigeration circuit extraction.The backflow of destilling tower, namely produces by any suitable method at the liquid of destilling tower flows downward inside.Such as, can backflow be provided, namely by overhead condenser with the indirect heat exchange of cooling agent and condensation at least partially overhead vapours obtain the backflow of condensate.Alternatively or extraly, the liquid backflow at the top of refluxing by being introduced into destilling tower provides.Cooling agent and/or liquid backflow can comprise the liquefied natural gas stream such as obtained from the liquefied natural gas produced by liquefaction system or produced.
Overhead vapours used herein or the stream removed from liquefaction system " are rich in " composition and (are such as rich in methane, nitrogen and/or another light composition), it means described overhead vapours or stream than to extract out from closed loop refrigeration circuit and the mix refrigerant being incorporated into the evaporation destilling tower has the concentration (molar percentage) of higher described composition.Similarly, the bottom liquid " being rich in " heavier composition means described bottom liquid than to extract out from closed loop refrigeration circuit and the mix refrigerant being introduced into the evaporation destilling tower has the concentration (molar percentage) of higher described composition.
The stream being rich in methane removed from liquefaction system discardable fall, or be positioned over any suitable object.It such as can be burned, as fuel (such as generation of power, electric power or effectively heating), is added into and is about to be liquefied in the natural gas feed of system liquefaction, or export (such as passing through pipeline) position to plant area.
When from destilling tower some or all bottom liquids stored before being again introduced into closed loop refrigeration circuit, bottom liquid can be stored in the bottom of destilling tower, and/or can extract out from destilling tower and be stored in independent reservoir vessel.In a preferred embodiment, produce all bottom liquids by destilling tower and to be again introduced in closed loop refrigeration circuit (directly and/or after temporarily storing).
The method removing cold-producing medium according to a first aspect of the invention preferably to liquefy to natural gas in response to liquefaction system and/or the closedown of excessively cold speed or idling and perform.Or the method can perform in response in other situation or disadvantageous situation, such as, when detecting in main heat exchanger or find to leak.
In the method for change productivity ratio according to a second aspect of the invention, first period such as can the normal operating of representative system, wherein the first productivity ratio is corresponding with the normal production rate of liquefaction or excessively cold natural gas, and the second period represented the cycle of idling or closedown, now the productivity ratio of liquefaction or excessively cold natural gas be reduced (to second or idling productivity ratio), or to be stopped together.
The method of change productivity ratio according to a second aspect of the invention can comprise for the second period further after another period or the 3rd period, during this period by the natural gas feeding by main heat exchanger is increased to the 3rd feed rate, cold-producing medium is added to liquefaction system, and the mix refrigerant of circulation is increased to the 3rd cycle rate, the productivity ratio of liquefaction or excessively cold natural gas is increased to the 3rd productivity ratio.Step cold-producing medium being added to liquefaction system can comprise and being introduced in closed loop refrigeration circuit by methane.Some or this methane all can obtain from natural gas supply, and natural gas supply provides natural gas, for liquefying in liquefaction system.If bottom liquid is not introduced in closed loop refrigeration circuit in the step (d) of the second period again (if or some bottom liquid store, and heavier composition still needs to be introduced in closed loop refrigeration circuit again) so for the step of liquefaction system interpolation cold-producing medium also can comprise, the bottom liquid of storage is introduced in closed loop refrigeration circuit again.3rd productivity ratio of liquefaction or excessively cold natural gas, the 3rd feed rate of natural gas and the 3rd cycle rate of mix refrigerant are preferably identical with the first cycle rate or less with the first productivity ratio, the first feed rate respectively.Specifically, the 3rd productivity ratio, the 3rd feed rate and the 3rd cycle rate can be identical with the first cycle rate with the first productivity ratio, the first feed rate respectively, and the 3rd period represented liquefaction system returns to normal operating.
Natural gas liquefaction system is according to a third aspect of the invention we particularly suitable for performing according to a first aspect of the invention and/or the method for second aspect.
Preferred aspect of the present invention comprises the aspect of following numbering #1 to #27:
#1. mono-kind removes the method for cold-producing medium from natural gas liquefaction system, natural gas liquefaction system uses mix refrigerant to liquefy to natural gas and/or excessively cold, mix refrigerant comprises the mixture of methane and one or more heavier compositions, and liquefaction system comprises closed loop refrigeration circuit, wherein mix refrigerant circulates when using liquefaction system, closed loop refrigeration circuit comprises main heat exchanger, natural gas transport passes through main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold, described method comprises:
A () extracts the mix refrigerant of evaporation out from closed loop refrigeration circuit;
B the mix refrigerant of evaporation is introduced in destilling tower by (), and provide backflow for destilling tower, thus the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane and the bottom liquid being rich in heavier composition;
C () extracts overhead vapours out from destilling tower, to form the stream being rich in methane, the stream being rich in methane removes from liquefaction system; With
D bottom liquid is introduced into closed loop refrigeration circuit from destilling tower by () again, and/or store bottom liquid, makes it follow-uply can be introduced in closed loop refrigeration circuit again.
The method of #2. according to aspect #1, is characterized in that, described heavier composition comprises one or more heavier hydrocarbon.
The method of #3. according to aspect #1 or #2, it is characterized in that, described mix refrigerant also comprises nitrogen, and the described overhead vapours in step (b) is rich in nitrogen and methane, and the stream being rich in methane described in step (c) is the stream being rich in nitrogen and methane.
The method of #4. according to the either side in aspect #1 to #3, it is characterized in that, in step (b), the backflow back up through condensate of described destilling tower provides, this by cool with the indirect heat exchange of cooling agent in overhead condenser with condensation at least partially overhead vapours obtain.
The method of #5. according to aspect #4, is characterized in that, described cooling agent comprises the liquefied natural gas stream obtained the liquefied natural gas produced from described liquefaction system or producing.
The method of #6. according to the either side in aspect #1 to #5, is characterized in that, in step (b), the liquid backflow back up through the top of introducing described destilling tower of described destilling tower provides.
The method of #7. according to aspect #6, is characterized in that, the backflow of described liquid comprises the liquefied natural gas stream obtained the liquefied natural gas produced from described liquefaction system or producing.
The method of #8. according to the either side in aspect #1 to #7, is characterized in that, the stream being rich in methane described in being formed in the step (c) is burned, as fuel and/or be added into and need to be liquefied in the natural gas feed of system liquefaction.
The method of #9. according to the either side in aspect #1 to #8, it is characterized in that, in step (d), described bottom liquid is stored in the bottom of described destilling tower, and/or extracts out from described destilling tower and be stored in independent reservoir vessel before being again introduced into described closed loop refrigeration circuit.
The method of #10. according to the either side in aspect #1 to #8, is characterized in that, in step (a), the mix refrigerant of described evaporation is from the cold junction of described main heat exchanger and/or extract out with centre position.
The method of #11. according to the either side in aspect #1 to #10, is characterized in that, described main heat exchanger is coil winding formula heat exchanger.
The method of #12. according to aspect #11, is characterized in that, in step (a), the mix refrigerant of described evaporation is extracted out from the shell-side of described coil winding formula heat exchanger.
The method of #13. according to the either side in aspect #1 to #12, is characterized in that, described method preferably to liquefy to natural gas in response to described liquefaction system and/or the closedown of excessively cold speed or idling and perform.
#14. mono-kind changes the method for the productivity ratio of liquefaction in natural gas liquefaction system and excessively cold natural gas, natural gas liquefaction system uses mix refrigerant to liquefy to natural gas and/or excessively cold, liquefaction system comprises closed loop refrigeration circuit, wherein mix refrigerant circulates in closed loop, mix refrigerant comprises the mixture of methane and one or more heavier compositions, and closed loop refrigeration circuit comprises main heat exchanger, natural gas transport passes through main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold, described method comprises:
First period, pass through main heat exchanger with the first feed rate transport gas during this period, and the cold-producing medium of mixing circulates with the first cycle rate in closed loop refrigeration circuit, thus produce liquefaction or excessively cold natural gas with the first productivity ratio;
Second period, during this period, by stopping by the natural gas feeding of main heat exchanger or its feed rate being reduced to the second feed rate, stop the circulation of mix refrigerant in closed loop refrigeration circuit or its cycle rate is reduced to the second cycle rate, and from liquefaction system, remove cold-producing medium, stop the production of liquefaction or excessively cold natural gas, or the productivity ratio of liquefaction or excessively cold natural gas is reduced to the second productivity ratio, and the method wherein removing cold-producing medium from liquefaction system comprises:
A () extracts the mix refrigerant of evaporation out from closed loop refrigeration circuit;
B the mix refrigerant of evaporation is introduced in destilling tower by (), and provide backflow for destilling tower, thus the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane and the bottom liquid being rich in heavier composition;
C () extracts overhead vapours out from destilling tower, to form the stream being rich in methane, the stream being rich in methane removes from liquefaction system; With
D bottom liquid is introduced into closed loop refrigeration circuit from destilling tower by () again, and/or store bottom liquid, makes it follow-uply can be introduced in closed loop refrigeration circuit again.
The method of #15. according to aspect #14, is characterized in that, after described second period, described method also comprises:
3rd period, during this period, by the natural gas feeding by described main heat exchanger is increased to the 3rd feed rate, cold-producing medium is added to described liquefaction system, and the mix refrigerant of circulation is increased to the 3rd cycle rate, the productivity ratio of liquefaction or excessively cold natural gas is increased to the 3rd productivity ratio, the step that wherein cold-producing medium is added to liquefaction system comprises and is introduced in described closed loop refrigeration circuit by methane, and if bottom liquid not to be introduced into again the words in described closed loop refrigeration circuit in the step (d) of the second period, just the bottom liquid of storage is introduced in described closed loop refrigeration circuit again.
The method of #16. according to aspect #15, it is characterized in that, the 3rd cycle rate of liquefaction or excessively cold the 3rd productivity ratio of natural gas, the 3rd feed rate of natural gas and mix refrigerant is identical with the first cycle rate or less with the first productivity ratio, the first feed rate respectively.
The method of #17. according to aspect #15 or #16, is characterized in that, the methane be introduced in described closed loop refrigeration circuit is supplied by natural gas and obtains, and natural gas supply provides for carrying out the natural gas liquefied in described liquefaction system.
The method of #18. according to the either side in aspect #15 to #17, is characterized in that, from the second period that described liquefaction system removes the method for cold-producing medium further limit by the either side aspect #2 to #12.
#19. mono-kind uses the natural gas liquefaction system of mix refrigerant to liquefy and/or crosses cold natural gas, and described mix refrigerant comprises the mixture of methane and one or more heavier compositions, and described liquefaction system comprises:
Closed loop refrigeration circuit, it is for holding cold-producing medium of mixing and making the refrigerant circulation of mixing when using liquefaction system, closed loop refrigeration circuit comprises main heat exchanger, natural gas can be conducted through main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold;
Destilling tower, it for receiving the mix refrigerant of the evaporation from closed loop refrigeration circuit, and can operate, the mix refrigerant of evaporation be separated into the overhead vapours being rich in methane of mix refrigerant and be rich in the bottom liquid of heavier composition;
For providing the device of backflow for destilling tower;
Conduit, it is for passing to destilling tower by the mix refrigerant of evaporation from closed loop refrigeration circuit, for extracting out from destilling tower and remove the stream being rich in methane formed by overhead vapours from liquefaction system, and for bottom liquid is introduced into closed loop refrigeration circuit from destilling tower again.
The system of #20. according to aspect #19, is characterized in that, described system also comprises storage device, and it for storing bottom liquid before bottom liquid is introduced into described closed loop refrigeration circuit again.
The system of #21. according to aspect #20, is characterized in that, the described storage device for storing bottom liquid comprises the base portion section of described destilling tower and/or independent reservoir vessel.
The system of #22. according to the either side in aspect #19 to #21, it is characterized in that, for providing the device of backflow to comprise overhead condenser for described destilling tower, it for being cooled by the indirect heat exchange with cooling agent and condensation overhead vapours at least partially, thus provides the backflow of condensate.
The system of #23. according to aspect #22, it is characterized in that, described cooling agent comprises liquefied natural gas stream, and described system also comprises conduit, and it is sent in described overhead condenser for a part of liquefied natural gas produced by described liquefaction system.
The system of #24. according to the either side in aspect #19 to #23, is characterized in that, for providing the device of backflow to comprise conduit for described destilling tower, it is for being introduced into the top of described destilling tower by liquid backflow.
The system of #25. according to aspect #24, is characterized in that, described liquid backflow comprises liquefied natural gas, and a part of liquefied natural gas that described liquefaction system is produced by the conduit for introducing described backflow is sent to the top of described destilling tower.
The system of #26. according to the either side in aspect #19 to #25, it is characterized in that, the conduit of the stream of methane is rich in by the described device given for the described stream that burns that spreads described in extracting out and removing, send to for the described stream that burns to produce the device of power or electric power, and/or sending natural gas feed conduit to, natural gas feed conduit is used for natural gas transport to liquefy to described liquefaction system.
The system of #27. according to the either side in aspect #19 to #26, it is characterized in that, for the conduit that the mix refrigerant of described evaporation passed to described destilling tower from described closed loop refrigeration circuit, the mix refrigerant of described evaporation is extracted out from the cold junction of described main heat exchanger and/or centre position.
The system of #28. according to the either side in aspect #19 to #27, is characterized in that, described main heat exchanger is coil winding formula heat exchanger.
The system of #29. according to aspect #28, is characterized in that, the mix refrigerant of described evaporation to be extracted out by the conduit for the mix refrigerant of described evaporation to be passed to described destilling tower from described closed loop refrigeration circuit from the shell-side of described coil winding formula heat exchanger.
As just example, now with reference to Fig. 1 to Fig. 6, some preferred embodiment of the present invention is described.In these figure all identical in feature is for a more than figure, this feature is in each figure for clear and designated for purpose of brevity identical label.
In embodiment shown in Fig. 1 to Fig. 6, natural gas liquefaction system has main heat exchanger, and it is coil winding formula heat exchanger type, and it comprises individual unit, wherein three independently tube bank be contained in identical shell, wherein natural gas is by tube bank, to be liquefied and excessively cold.But, should understand, more or less tube bank can be used, and restrain (when using a more than tube bank) and can be contained in independently in shell, make main heat exchanger will comprise a series of unit.Comparably, main heat exchanger needs not be coil winding type, and is alternately the heat exchanger of another type, such as but be not limited to shell and tube heat exchanger or the fin type heat exchanger of another type.
Equally, in embodiment shown in Fig. 1 to Fig. 6, natural gas liquefaction system utilizes C3MR to circulate or DMR circulation, to liquefy to natural gas with excessively cold, correspondingly arrange in figure and depict and hold the closed loop refrigeration circuit of mix refrigerant, it is for liquefaction and cross cold natural gas (wherein for the pre-cooled portion section not showing propane or mix refrigerant for the purpose of simple).But, the refrigerant circulation of other type can be used equally, such as but be not limited to SMR circulation or C3MR-nitrogen mixing circulation.In this alternative circulation, the cold-producing medium of mixing only for making natural gas liquefaction or excessively cold, and correspondingly may reconfigure closed loop refrigeration circuit, and the cold-producing medium wherein mixed circulates at closed loop refrigeration circuit.
In these embodiments, mix refrigerant comprises methane and one or more heavier compositions.Heavier composition preferably includes one or more heavier hydrocarbon, and nitrogen also exists as extra light composition.Specifically, preferred mix refrigerant comprises the mixture of nitrogen, methane, ethane/ethylene, propane, butane and pentane usually.
With reference to Fig. 1, show the natural gas liquefaction system operated during the first period according to an embodiment of the invention, wherein it operates under normal operation, during this period, natural gas is conducted through main heat exchanger with the first feed rate, and the cold-producing medium of mixing circulates with the first cycle rate in closed loop refrigeration circuit, thus produce liquefaction and excessively cold natural gas with the first productivity ratio or normal production rate.For the purpose of simple, do not describe the feature for removing the liquefaction system of cold-producing medium under follow-up idling or closedown condition from liquefaction system in FIG, and it will be described in further detail with reference to Fig. 2 to Fig. 4 below.
Natural gas liquefaction system comprises closed circuit refrigerating circuit, and it comprises main heat exchanger 10, coolant compressor 30 and 32, refrigerant cooler 31 and 32, phase separator 34 and expansion gear 36 and 37 in this example.Main heat exchanger 10 is coil winding formula heat exchanger as mentioned above, and it comprises the tube bank 11,12,13 of three spiral windings, and it is contained in the shell (being usually made up of aluminium or stainless steel) of single pressurization.Each tube bank can be made up of several thousand pipelines, is wrapped in around central shaft in a spiral manner, and is connected on tube sheet, tube sheet be positioned at tube bank above and below.
In this embodiment, natural gas feed flow 101 is cooled in the pre-cooled portion section (not shown) of liquefaction system, pre-cooled portion section uses propane or mix refrigerant to carry out pre-cooled to natural gas in different closed loops, natural gas feed flow 101 enters in the hot junction of coil winding formula heat exchanger 10 and flows through the hot junction 11 of tube bank, mid portion 12 and cold junction 13 along with it and carry out liquefying and excessively cold, flows 102 afterwards and leave the cold junction of coil winding formula heat exchanger as excessively cold liquefied natural gas (LNG).If necessary, natural gas feed flow 101 also will carry out pretreatment, any moisture, sour gas, mercury and natural gas liquids (NGL) are removed the level to necessity, thus avoid solidifying or other operational issue in coil winding formula heat exchanger 10.The excessively cold liquefied natural gas (LNG) leaving coil winding formula heat exchanger flows 102 directly can send to pipeline (not shown) for being sent to off-site, and/or LNG storage tank 14 can be sent to, can therefrom extract liquefied natural gas 103 out when needs.
Natural gas in coil winding formula heat exchanger by with cold evaporation or the indirect heat exchange of mix refrigerant of evaporating and carrying out cool, to liquefy and excessively cold, cold evaporation or the mix refrigerant that evaporating flow to hot junction in the outside of pipeline from cold junction by the shell-side of coil winding formula heat exchanger.The top of each tube bank in shell is positioned with distributor assembly usually, and it makes shell-side cold-producing medium across tube bank distribution.
The mix refrigerant 309 leaving the evaporation of warming in coil winding formula heat exchanger hot junction compresses in coolant compressor 30 and 32, and carry out cooling (cooling agent being generally water or another environment temperature) in intercooler and aftercooler 31 and 33, thus form the mixed refrigerant stream 312 of the partial condensation of compression.Then this stream is separated into the liquid stream of mix refrigerant 301 and the steam stream of mix refrigerant 302 at phase separator 34.In an illustrated embodiment, coolant compressor 30 and 32 is driven by common motor 35.
The relative natural gas feed flow 101 of liquid stream of mix refrigerant 301 is independently through hot junction 11 and the mid portion 12 of coil winding formula heat transfer tube bundle, thus also cool there, expand in expansion gear 36 afterwards, to form cold flow of refrigerant 307, its typical temperature is approximately-60 to-120 DEG C, centre position between its cold junction 13 in tube bank and mid portion 12 is introduced into the shell-side of coil winding formula heat exchanger 10 again, thus the aforesaid cold evaporation of part or the mix refrigerant evaporated are provided, it flows through the shell-side of coil winding formula heat exchanger.
The relative natural gas feed flow 101 of steam stream of mix refrigerant 302 is independently through the hot junction 11 of coil winding formula heat transfer tube bundle, mid portion 12 and cold junction 13, thus also carry out cooling there and condensation at least in part, expand in expansion gear 37 afterwards, to form cold flow of refrigerant 308, its typical temperature is approximately-120 to-150 DEG C, it is introduced into the shell-side of coil winding formula heat exchanger 10 again at the cold junction of coil winding formula heat exchanger, thus the remainder of the mix refrigerant aforesaid cold evaporation of part being provided or evaporating, it flows through the shell-side of coil winding formula heat exchanger.
Should be realized that, word " heat " above and " cold " only refer to the relative temperature of indication stream or parts, unless and point out, otherwise any special temperature range can not be contained.In the embodiment shown in fig. 1, expansion gear 36 and 37 is Joule-Thomson (J-T) valves, but can use any device that other is suitable for making the flow of refrigerant of mixing to expand comparably.
With reference to Fig. 2, now illustrate the natural gas liquefaction system operated during the second period, wherein it operates under idling or closedown condition, during this period, the production of liquefaction and excessively cold natural gas is reduced or stops, and cold-producing medium removes now from natural gas liquefaction system.
When liquefaction system operates with idling condition, natural gas feed flow 101 is still through coil winding formula heat exchanger 10, to produce cold liquefied natural gas stream 102, but with the feed rate in Fig. 1 and productivity ratio comparatively, the feed rate (i.e. the flow velocity of natural gas feed flow 101) of natural gas and the productivity ratio (flow velocity of namely excessively cold liquefied natural gas stream 102) of liquefied natural gas are reduced.Similarly, compare with the cycle rate in Fig. 1, the cycle rate (namely around loop and especially through the flow velocity of the mix refrigerant of main heat exchanger 10) of the mix refrigerant in closed loop refrigeration circuit is reduced, thus decrease the cooling task amount that cold-producing medium provides, to mate the productivity ratio of the liquefied natural gas of minimizing.When liquefaction system operates with cut out condition, the production of the feeding of natural gas, the circulation of mix refrigerant and (certainly) excessively cold liquefied natural gas is all stopped.
By extracting out from the shell-side of coil winding formula heat exchanger 10 in its cold junction, the mixed refrigerant stream 201 of evaporation is extracted out from closed loop refrigeration circuit, and be introduced into the bottom of destilling tower 20, destilling tower comprises multiple separation level be made up of such as filler or pallet, and it is for being separated into the overhead vapours being accumulated in destilling tower top and the bottom liquid be accumulated in bottom destilling tower by the mix refrigerant of evaporation.For the mix refrigerant be transported in tower, overhead vapours is rich in the methane of mix refrigerant and other light composition any.Such as, when nitrogen is present in the cold-producing medium of mixing, overhead vapours is also rich in nitrogen.For the mix refrigerant be transported in tower, bottom liquid is rich in the composition of the mix refrigerant heavier than methane.As described above, typical heavier composition comprises such as ethane/ethylene, propane, butane and pentane.The operating pressure of destilling tower is less than 150psig (being less than 100atm) usually.
In this embodiment, destilling tower back up through in overhead condenser 22 with the indirect heat exchange of cooling agent 207 and cooling condensation at least partially overhead vapours produce.Overhead condenser 22 can form overall with the top of destilling tower 20 or it is a part of, or it may (as shown in Figure 2) be the separate unit being passed overhead vapours.
Overhead vapours 202 from destilling tower 20 passes condenser 22, and in this embodiment, partly condensation is to form mixed phase flow 203.Then mixed phase flow 203 is separated into liquid condensate and remains the vapor portion being rich in methane in phase separator 21, liquid condensate returns destilling tower as backflow 210, and the vapor portion that residue is rich in methane removes from liquefaction system as the stream 204 being rich in methane.(not shown) in an alternative embodiment, overhead vapours 202 can in overhead condenser total condensation, and the overhead fraction of condensation is split into two streams, one of them stream returns to the top of destilling tower as backflow 210, and its another stream forms the stream 204 being rich in methane that (being liquid in this case) extracts out from liquefaction system.This will allow that phase separator 21 is removed, but also increase needing the cooling task being used for overhead condenser, and not be therefore preferred usually.
The stream 204 being rich in methane extracted out from liquefaction system is preferably main not containing heavier composition.Such as, when heavier composition comprises ethane and higher hydrocarbon, it comprises these compositions being less than about 1% usually.Also deposit in the case of nitrogen in mix refrigerant, stream 204 is rich in methane and nitrogen.Nitrogen in stream will depend on their ratios in the mix refrigerant of the evaporation of extracting out from closed loop refrigeration circuit to the ratio of methane, but usually will at about 5-40 % by mole N 2scope in.The stream 204 being rich in methane is by send in the device of torch rack (not shown) or other this stream that is appropriate to burn and to carry out burning and discard, but preferably used as fuel, send to exterior tubing or outside natural gas motorcar, or be added to natural gas feed flow 101, thus provide extra charging, for producing extra excessively cold liquefied natural gas.If the stream 204 being rich in methane is used as fuel, it can such as burn in the burner of gas turbine (not shown) or other form, to produce the power (such as starting refrigerant condenser 30 and 32 by motor 35) being used for onsite application, thus the electric power produced for exporting, and/or in the factory, such as, in acid gas removal body unit, provide technique to heat.
Bottom liquid 221/222 is introduced into closed loop refrigeration circuit from destilling tower 20 again, and/or stores, and makes it follow-uply can be introduced in closed loop refrigeration circuit again.As mentioned above, bottom liquid is rich in heavier composition, and is preferably grouped into primarily of these heavier one-tenth.It preferably comprises the methane that is less than 10 % by mole and other light composition any (such as, is less than the CH of 10 % by mole 4+ N 2).It can be introduced in closed loop refrigeration circuit in any suitable position again.Such as, bottom liquid 221 can be introduced into the same position of the coil winding formula heat exchanger of the mix refrigerant (utilizing such as identical conduit) therefrom extracting evaporation out again, or as shown in Figure 2, it in the centre position of heat exchanger, such as, can be reintroduced to the shell-side of coil winding formula heat exchanger 10 between the cold junction 13 and mid portion 12 of tube bank.When some or all bottom liquids stored before being reintroduced in coil winding formula heat exchanger 10, bottom liquid 222 can be stored in relative destilling tower independently in reservoir vessel, in recovery drum 24 such as shown in Fig. 2, or the bottom of destilling tower 20 itself can be designed for temporary reservoir bottom liquid.If needed, not that all bottom liquids that destilling tower produces all need to be introduced in closed loop refrigeration circuit again, and/or store, be introduced into again in closed loop refrigeration circuit for follow-up.But it is preferred for usually introducing all bottom liquids (and/or store, then follow-up introduce again) again.
As discussed above, by introducing again (or store, introduce again afterwards) bottom liquid gets back in closed loop refrigeration circuit, the heavier composition (such as ethane/ethylene and higher hydrocarbon) of mix refrigerant can remain, thus avoid the needs changing these compositions once the normal operating recovering liquefaction system in the cold-producing medium of mixing, this may be expensive, difficulty, and is operation consuming time.Simultaneously, by remove from destilling tower and liquefaction system by overhead vapours formed be rich in methane stream (simply by burn this stream or by being placed on other purposes), also avoid the difficulty be associated with the methane and other extra light composition (such as nitrogen) any that store mix refrigerant.
Can from any suitable source for the cooling agent in overhead condenser.Such as, if can obtain at the scene, the nitrogen (LIN) of liquefaction so can be used to flow.But in a preferred embodiment, as shown in Figure 2, liquefied natural gas is used as cooling agent.Liquefied natural gas directly can take from the liquefied natural gas (if system operates under idling condition) that liquefaction system is being produced, or as shown in the figure, it can pump out from LNG storage tank 14.The liquefied natural gas stream 209/207 extracted out from accumulator tank 14 was pumped overhead condenser 22 as cooling agent by pump 23.Liquefied natural gas stream warms in overhead condenser, and leaves condenser as the natural gas flow 208 warmed, and it such as can burn, or is used as fuel according to the mode similar to the stream 204 being rich in methane discussed above.If the natural gas flow warmed 208 is two-phases, so it can send it back in LNG storage tank 14 or separator (not shown), therefrom liquid can send to liquefied natural gas groove, and steam can carry out burning or be used as fuel or cold-producing medium supply or for some other purposes, as described in before for overhead vapours.
The flow-control (other embodiments of the invention) of various stream depicted in figure 2 realizes by any and all suitable devices in this area.Such as, flow to the flow-control of the mix refrigerant 201 of the evaporation of destilling tower, the flow-control of flow-control that bottom liquid 221 flows back to coil winding formula heat exchanger and the stream 204 that is rich in methane realizes by the supravasal one or more suitable volume control devices (such as flow control valve) being positioned at one or more transmission or extracting these streams out.Similarly, the flow of liquefied natural gas stream 209/207 can utilize volume control device such as flow control valve to control, when usual pump 23 itself will provide appropriate flow-control.
As mentioned above, in the embodiment shown in Figure 2, provide the backflow of destilling tower, it is by condensation overhead vapours and the condensate obtained at least partially.But substitute (or except) condensation overhead vapours, the backflow of destilling tower alternately (or extraly) is provided by direct independent liquid stream to be injected in destilling tower.Shown by this has in figure 3, operate under being wherein in idling or closedown condition according to the natural gas liquefaction system display of the present invention's alternative.
With reference to Fig. 3, the mixed refrigerant stream 201 of evaporation is extracted out from the shell-side of coil winding formula heat exchanger 10 in its cold junction again, and being incorporated into the bottom of destilling tower 20, the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane (with other light composition any) and the bottom liquid being rich in heavier composition by destilling tower 20 again.But, in this embodiment, there is no overhead condenser and the separator be associated for providing backflow for destilling tower.On the contrary, the liquefied natural gas stream 209/207 pumped from LNG storage tank 14 is incorporated into the top of destilling tower as backflow, and the overhead vapours that all tops from destilling tower are extracted out forms the stream 204 being rich in methane, the stream 204 being rich in methane is extracted out from liquefaction system (and it as described abovely burns, as fuel, be added to natural gas feed or send to pipeline).
Equally, in the embodiment shown in fig. 3, substitute or supplementary liquefied natural gas, in available situation, other suitable cooled liquid stream can be used to provide the backflow of destilling tower.Such as, LIN stream can again for alternative liquefied natural gas stream.But, when liquid stream is incorporated in destilling tower, when making it occur directly to contact with the cold-producing medium of the mixing being included in the inside, the composition of liquid stream should such as unacceptable ground contamination bottom liquid 221/222, bottom liquid 221/222 or return in closed loop refrigeration circuit by follow-up, as the cold-producing medium retained.Specifically, if liquid stream comprises any composition of the pollutant in composition mix refrigerant, so this composition should have sufficiently high volatility, and/or should exist with enough low amount, and the amount making the described composition the bottom liquid extracted out from destilling tower is inessential.
In another embodiment, embodiment shown in Fig. 2 and Fig. 3 can combine, make destilling tower back up through the condensate formed by the overhead vapours of carrying out condensation in overhead condenser, and to be provided by direct independent liquid stream is injected in destilling tower.
In the embodiment shown in Fig. 2 and Fig. 3, to extract out and the mixed refrigerant stream 201 being incorporated into the evaporation destilling tower 20 extracts out from the shell-side of coil winding formula heat exchanger 10 in cold junction from closed-loop refrigeration system.But in an alternative embodiment, the mixed refrigerant stream of evaporation can be extracted out from the another location of closed loop refrigeration circuit.
For example, referring to Fig. 4, which show according to another embodiment of the present invention be in idling or closedown condition under carry out the natural gas liquefaction system that operates.In this embodiment, the mixed refrigerant stream 201 of evaporation is still extracted out from the shell-side of coil winding formula heat exchanger 10 and is introduced into the bottom of destilling tower 20.Similarly, the shell-side of coil winding formula heat exchanger 10 again again can be introduced into from the bottom liquid 221 of destilling tower 20.But, in this embodiment, the mixed refrigerant stream 201 of evaporation extracts out from the centre position of heat exchanger, such as between the cold junction 13 and mid portion 12 of tube bank, and bottom liquid is the shell-side from coil winding formula heat exchanger, closer to the position towards heat exchanger hot junction, such as, return between the mid portion 12 and hot junction 11 of tube bank.
With reference to Fig. 5 and Fig. 6, show the present according to an embodiment of the invention natural gas liquefaction system carrying out operating during the 3rd period, during this period, the production of liquefaction and excessively cold natural gas increases (after the operation after shutdown or under idling condition), and return to normal production rate, and cold-producing medium is being introduced in natural gas liquefaction system again.For the purpose of simple, do not describe the feature removing the liquefaction system of cold-producing medium for liquefaction system under idling or closedown condition in fig. 5 and fig., such as, above with reference to the destilling tower 20 described in Fig. 2 to Fig. 4 and overhead condenser 22 (in a situation of use where).
During enabling, be increased, until again reach normal production rate through the feed rate (i.e. the flow velocity of natural gas feed flow 101) of the natural gas of coil winding formula heat exchanger 10 and the productivity ratio (flow velocity of namely excessively cold liquefied natural gas stream 102) of liquefied natural gas that obtains.Similarly, the cycle rate (namely around loop and especially through the flow velocity of the mix refrigerant of main heat exchanger 10) of the mix refrigerant in closed loop refrigeration circuit is increased, thus the cooling task of increase is provided, this increases required for liquefied natural gas (LNG) production rate.In order to provide this increase in the cycle rate of mix refrigerant, cold-producing medium must be added back closed loop refrigeration circuit, thus provide supply for the cold-producing medium removed when liquefaction system operates under idling or cut out condition before.
In the embodiment shown in Fig. 5 and Fig. 6, when liquefaction system close or when operating under idling condition before the period, bottom liquid from destilling tower is stored in and reclaims in drum 24, and the supplementary cold-producing medium comprising the heavier composition of mix refrigerant needs again to be introduced in closed loop refrigeration circuit now.Similarly, in these embodiments, cold-producing medium is introduced to get back in closed loop refrigeration circuit again and is related to from reclaiming the bottom liquid 401 extracting storage drum 24 out, and is introduced in closed loop refrigeration circuit by described bottom liquid again.Above with reference to described in Fig. 2 to Fig. 4, bottom liquid can be introduced in any suitable position again and get back in closed loop refrigeration circuit.Such as, as shown in Figure 5, expand by expansion gear such as J-T valve 40 from the bottom liquid 401 reclaiming drum 24 extraction, and be introduced into the shell-side of coil winding formula heat exchanger again near its cold junction.Or, as shown in Figure 6, can expand from reclaiming the bottom liquid 401 that drum 24 extracts out, and in the downstream of coolant compressor 30 and 32 and aftercooler 33, and introduce again in closed loop refrigeration circuit in the upstream of cold-producing medium phase separator 34.In both cases, demand bottom liquid being introduced into again the pump in closed loop refrigeration circuit is avoided by allowing that the pressure reclaiming drum 24 is increased on the operating pressure introduced again a little.
Cold-producing medium is introduced to get back in closed loop refrigeration circuit again and usually needs is added methane and other light composition any, such as nitrogen, it is designed to be present in the cold-producing medium of mixing, and removes from liquefaction system as stream 204 part being rich in methane during the cycle of idling or shutoff operation.May preferably, before bottom liquid 401 is introduced from recovery drum 24 again getting back to closed-loop refrigeration system, methane and other light refrigerant any are introduced in closed-loop refrigeration system.Supply methane (with other light composition any) can obtain from any suitable source, and can be introduced in closed-loop refrigerant circuits in any suitable position.
Specifically, when natural gas mainly methane (usually about 95 % by mole) time, provide the natural gas of natural gas feed flow 101 to supply as closed loop refrigeration circuit is provided convenience and easy methane Source Of Supply.As mentioned above, before being introduced in coil winding formula heat exchanger and liquefying, natural gas feed carries out washing to remove NGL usually.These liquefied natural gas process usually in GL fractionating system (not shown), and it comprises a series of destilling tower, comprise demethanizer or scrubbing tower, and it produces the tower top fraction being rich in methane.Such as, this tower top fraction being rich in methane can be used as supply methane 402, and it such as can be added to closed loop refrigeration circuit in the upstream of the downstream of coil winding formula heat exchanger and the first coolant compressor 30.
example
Operation in order to demonstrate the invention, the technique removing cold-producing medium from natural gas liquefaction system utilizing described in ASPEN+ software simulation Fig. 2 and describe.
The baseline of this example is the liquefied natural gas (LNG) facilities of 500 ten thousand public ton/year (mtpa) utilizing C3MR to circulate, and it produces the liquefied natural gas of about 78,000lbmoles/h (35380kgmoles/h).This example is that interchanger reinstates several hours, until set up the closedown situation of the pressure of 100psi (6.8atm) due to approximately ~ 130kbtu/hr (38kW) heat leak.Emulation represents the initial operation of destilling tower 20.List the condition of stream in the following table.For this example, destilling tower is 0.66ft (20cm) diameter, and 15ft (4.57m) is long, and " the filler of (2.5cm) Pall ring form that comprises employing 1.It is effective for these results show destilling tower in the light composition (methane and nitrogen) being separated mix refrigerant and heavier composition (ethane/ethylene, propane and butane), and thus effectively retains in the prolongation down periods and reclaimed described valuable heavier composition.
Should understand, the present invention is not limited to above with reference to the details described in preferred embodiment, can make many modifications and variations not departing from the spirit and scope of the present invention defined by the following claims on the contrary.

Claims (28)

1. one kind removes the method for cold-producing medium from natural gas liquefaction system, described liquefaction system uses mix refrigerant to liquefy to natural gas and/or excessively cold, described mix refrigerant comprises the mixture of methane and one or more heavier compositions, and described liquefaction system comprises closed loop refrigeration circuit, wherein said mix refrigerant circulates when using described liquefaction system, described closed loop refrigeration circuit comprises main heat exchanger, natural gas transport is by described main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold, described method comprises:
A () extracts the mix refrigerant of evaporation out from described closed loop refrigeration circuit;
B the mix refrigerant of evaporation is introduced in destilling tower by (), and provide backflow for described destilling tower, thus the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane and the bottom liquid being rich in heavier composition;
C () extracts overhead vapours out from described destilling tower, to form the stream being rich in methane, described in be rich in methane stream remove from described liquefaction system; With
D bottom liquid is introduced into described closed loop refrigeration circuit from described destilling tower by () again, and/or store bottom liquid, makes it follow-uply can be introduced in described closed loop refrigeration circuit again.
2. method according to claim 1, is characterized in that, described heavier composition comprises one or more heavier hydrocarbon.
3. method according to claim 1, it is characterized in that, described mix refrigerant also comprises nitrogen, and the described overhead vapours in step (b) is rich in nitrogen and methane, and the stream being rich in methane described in step (c) is the stream being rich in nitrogen and methane.
4. method according to claim 1, it is characterized in that, in step (b), the backflow back up through condensate of described destilling tower provides, described condensate back up through cool with the indirect heat exchange of cooling agent in overhead condenser with condensation at least partially overhead vapours obtain.
5. method according to claim 4, is characterized in that, described cooling agent comprises the liquefied natural gas stream obtained the liquefied natural gas produced from described liquefaction system or producing.
6. method according to claim 1, is characterized in that, in step (b), the liquid backflow back up through the top of introducing described destilling tower of described destilling tower provides.
7. method according to claim 6, is characterized in that, the backflow of described liquid comprises the liquefied natural gas stream obtained the liquefied natural gas produced from described liquefaction system or producing.
8. method according to claim 1, is characterized in that, the stream being rich in methane described in being formed in the step (c) is burned, as fuel and/or be added into and need to be liquefied in the natural gas feed of system liquefaction.
9. method according to claim 1, it is characterized in that, in step (d), described bottom liquid is stored in the bottom of described destilling tower, and/or extracts out from described destilling tower and be stored in independent reservoir vessel before being again introduced into described closed loop refrigeration circuit.
10. method according to claim 1, is characterized in that, in step (a), the mix refrigerant of described evaporation is from the cold junction of described main heat exchanger and/or extract out with centre position.
11. systems according to claim 1, is characterized in that, described main heat exchanger is coil winding formula heat exchanger.
12. methods according to claim 11, is characterized in that, in step (a), the mix refrigerant of described evaporation is extracted out from the shell-side of described coil winding formula heat exchanger.
13. methods according to claim 1, is characterized in that, described method preferably to liquefy to natural gas in response to described liquefaction system and/or the closedown of excessively cold speed or idling and perform.
14. 1 kinds of methods changing the productivity ratio of liquefaction in natural gas liquefaction system and excessively cold natural gas, described liquefaction system uses mix refrigerant to liquefy to natural gas and/or excessively cold, described liquefaction system comprises closed loop refrigeration circuit, wherein said mix refrigerant circulates in described closed loop, described mix refrigerant comprises the mixture of methane and one or more heavier compositions, and described closed loop refrigeration circuit comprises main heat exchanger, natural gas transport is by described main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold, described method comprises:
First period, described main heat exchanger is passed through during this period with the first feed rate transport gas, and described mix refrigerant circulates with the first cycle rate in described closed loop refrigeration circuit, thus produce liquefaction or excessively cold natural gas with the first productivity ratio;
Second period, during this period, by stopping by the natural gas feeding of described main heat exchanger or its feed rate being reduced to the second feed rate, stop the circulation of described mix refrigerant in closed loop refrigeration circuit or its cycle rate is reduced to the second cycle rate, and from described liquefaction system, remove cold-producing medium, stop the production of liquefaction or excessively cold natural gas, or the productivity ratio of liquefaction or excessively cold natural gas is reduced to the second productivity ratio, and the method wherein removing cold-producing medium from described liquefaction system comprises:
A () extracts the mix refrigerant of evaporation out from described closed loop refrigeration circuit;
B the mix refrigerant of evaporation is introduced in destilling tower by (), and provide backflow for described destilling tower, thus the mix refrigerant of evaporation is separated into the overhead vapours being rich in methane and the bottom liquid being rich in heavier composition;
C () extracts overhead vapours out from described destilling tower, to form the stream being rich in methane, described in be rich in methane stream remove from described liquefaction system; With
D bottom liquid is introduced into described closed loop refrigeration circuit from described destilling tower by () again, and/or store bottom liquid, makes it follow-uply can be introduced in described closed loop refrigeration circuit again.
15. methods according to claim 14, is characterized in that, described method also comprises after described second period:
3rd period, during this period, by the natural gas feeding by described main heat exchanger is increased to the 3rd feed rate, cold-producing medium is added to described liquefaction system, and the circulation of mix refrigerant is increased to the 3rd cycle rate, the productivity ratio of liquefaction or excessively cold natural gas is increased to the 3rd productivity ratio, the step wherein cold-producing medium being added to liquefaction system comprises and is introduced in described closed loop refrigeration circuit by methane, and if bottom liquid not to be introduced into again the words in described closed loop refrigeration circuit in the step (d) of the second period, just the bottom liquid of storage is introduced in described closed loop refrigeration circuit again.
16. methods according to claim 15, it is characterized in that, the 3rd cycle rate of liquefaction or excessively cold the 3rd productivity ratio of natural gas, the 3rd feed rate of natural gas and mix refrigerant is identical or less with described first cycle rate with described first productivity ratio, described first feed rate respectively.
17. methods according to claim 15, is characterized in that, the methane be introduced in described closed loop refrigeration circuit obtains from natural gas supply, and described natural gas supply provides for carrying out the natural gas liquefied in described liquefaction system.
18. 1 kinds use mix refrigerant to liquefy to natural gas and/or excessively cold natural gas liquefaction system, and described mix refrigerant comprises the mixture of methane and one or more heavier compositions, and described liquefaction system comprises:
Closed loop refrigeration circuit, it is for holding cold-producing medium of mixing and making the refrigerant circulation of mixing when using described liquefaction system, described closed loop refrigeration circuit comprises main heat exchanger, natural gas can be conducted through described main heat exchanger, to be liquefied by the indirect heat exchange and carrying out of the mix refrigerant with circulation and/or excessively cold;
Destilling tower, it for receiving the mix refrigerant of the evaporation from closed loop refrigeration circuit, and can operate, the mix refrigerant of evaporation to be separated into the bottom liquid of the overhead vapours being rich in methane and the heavier composition being rich in mix refrigerant;
For providing the device of backflow for described destilling tower;
Conduit, it is for passing to described destilling tower by the mix refrigerant of evaporation from described closed loop refrigeration circuit, for extracting out from described destilling tower and remove the stream being rich in methane formed by overhead vapours from described liquefaction system, and for bottom liquid is introduced into described closed loop refrigeration circuit from described destilling tower again.
19. systems according to claim 18, is characterized in that, described system also comprises storage device, and it for storing bottom liquid before bottom liquid is introduced into described closed loop refrigeration circuit again.
20. systems according to claim 19, is characterized in that, the described storage device for storing bottom liquid comprises the base portion section of described destilling tower and/or independent reservoir vessel.
21. systems according to claim 18, it is characterized in that, for providing the device of backflow to comprise overhead condenser for described destilling tower, it for being cooled by the indirect heat exchange with cooling agent and condensation overhead vapours at least partially, thus provides the backflow of condensate.
22. systems according to claim 21, it is characterized in that, described cooling agent comprises liquefied natural gas stream, and described system also comprise conduit, and it is sent to described overhead condenser for a part of liquefied natural gas produced by described liquefaction system.
23. systems according to claim 18, is characterized in that, for providing the device of backflow to comprise conduit for described destilling tower, it is for being introduced into liquid backflow in the top of described destilling tower.
24. systems according to claim 23, is characterized in that, described liquid backflow comprises liquefied natural gas, and a part of liquefied natural gas that described liquefaction system is produced by the conduit for introducing described backflow is sent in the top of described destilling tower.
25. systems according to claim 18, it is characterized in that, the conduit of the stream of methane is rich in by the described device given for the described stream that burns that spreads described in extracting out and removing, send to for the described stream that burns to produce the device of power or electric power, and/or sending natural gas feed conduit to, described natural gas feed conduit is used for natural gas transport to liquefy to described liquefaction system.
26. systems according to claim 18, it is characterized in that, for the conduit that the mix refrigerant of described evaporation passed to described destilling tower from described closed loop refrigeration circuit, the mix refrigerant of described evaporation is extracted out from the cold junction of described main heat exchanger and/or centre position.
27. systems according to claim 18, is characterized in that, described main heat exchanger is coil winding formula heat exchanger.
28. systems according to claim 27, it is characterized in that, the mix refrigerant of described evaporation to be extracted out by the conduit for the mix refrigerant of described evaporation to be passed to described destilling tower from described closed loop refrigeration circuit from the shell-side of described coil winding formula heat exchanger.
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